Particularly in the case of large widths of the coating machine and at high speeds, the method of curtain coating shows a great number of problems with respect to the uniform application of the coating liquids present in the free falling curtain to the moving web.
There is the problem that the edges of the free falling curtain need to be stabilized, because otherwise the curtain would contract towards the center under the influence of surface tension forces. Lateral guides are normally used in order to obtain a stable curtain that has the same thickness over the whole coating width. Different types of such lateral guides have been described for example in patent applications EP 0,281,520, EP 0,606,038, EP 0,740,197, EP 0,841,588, EP 0,907,103 and EP 1,023,949. A supplementary liquid (lateral flow liquid) is supplied at the edges of the curtain in order to reduce the deceleration of the rate of fall induced by the friction of the curtain liquid at the lateral guides. The edge zones of the curtain are cut off and/or sucked off by a vacuum device before the curtain impinges on the web in order to adjust the coating width and to stabilize the border areas formed on the web by the impinging coating solutions.
All these methods forming the state of the art are based on the assumption that the lateral flow liquid added at the lateral guides has a lubricating and rinsing effect which stabilizes the free fall of the curtain. There is the further assumption that this supplementary amount of lateral flow liquid has to be separated before the curtain impinges on the moving web. All these separation devices however, in particular the suction based devices equipped with small orifices, are prone to obstruction and contamination, inducing different problems during the coating process, particularly in the case where the curtain contains coating solutions that harden quickly, are sticky or reactive and/or in the case of high coating weights.
New lateral guides are described in patent application EP 0,740,197, where the lateral flow liquid, for example water or a mixture of water and glycerol, is supplied as a film transversely to the extension of the curtain and where this liquid film flows downwards in a groove of the lateral guides. This prevents uncontrolled undulating disturbances in the curtain. There are no indications in this patent application on the width of the groove or on its surface properties best suited for the coating process.
A suction device that may be used together with these lateral guides is described in patent application EP 0,841,588, wherein the border area of the curtain is removed together with the added lateral flow liquid. Without this suction device the lateral guides described in patent application EP 0,740,197 may not be used in regular production curtain coating operations.